Various accurate 2-dimensional (2D) optical position sensing systems are known. For example, one 2D incremental position sensor using a 2D grating scale and providing high resolution and high accuracy for sensing translation in an X-Y plane is disclosed in U.S. Pat. No. 5,104,225 to Masreliez, which is incorporated herein by reference in its entirety. Such a system is essentially an orthogonal combination of well known 1-dimensional (1D) optical encoder “incremental” measurement techniques that sense the position of a readhead within a particular period of a periodic scale grating for high resolution and continuously increment and decrement a count of the number of periods of the periodic scale that are traversed during a series of movements, in order to continuously provide a net relative displacement between the readhead and scale. However, such systems cannot sense the “z-axis” separation between a readhead and scale.
A very limited number of types of optical position sensors capable of sensing more than two degrees of freedom of a relative position of an object are known. One system comprising a probe that can sense relative position for up to 6 degrees of freedom is disclosed in U.S. Pat. No. 5,453,838 to Danielian and Neuberger. The '838 patent discloses a probe using a fiber optic bundle, with individual fibers or sets of fibers acting as individual intensity sensing channels. The individual intensity signals vary with X-Y motion of an illuminated target surface, as well as with the proximity of each fiber to the illuminated target surface along a direction normal to the surface. However, the probe disclosed in the '838 patent provides relatively crude measurement resolution and a limited sensing range for “z-axis” separation and orientation between the probe and a target surface.
Known dual-camera “stereoscopic” triangulation systems can sense relative position for up to 6 degrees of freedom. However, such known dual-camera systems are generally relatively large systems developed for measuring macroscopic objects and/or their positions, which do not scale well to relatively compact precision position measuring systems usable in close proximity to their target object. Furthermore, the triangulation arrangement of such known systems generally constrains the relationship between the z-axis measurement resolution and the z-axis measurement range in a restrictive and undesirable manner.
Systems that can image an object and determine x-y position from a feature in the image and z-axis position and orientation based on varying magnification in the image are also known. However, the magnification arrangement of such known systems generally constrains the relationship between the z-axis measurement resolution and the z-axis measurement range in a restrictive and undesirable manner, and introduces other problems requiring special image processing and/or compensation in order to accurately measure a relative position with up to 6 degrees of freedom.